The goal of this project is to study the cellular basis of active sensation. A crucial function of all nervous systems is to distinguish between sensory stimuli originating from the external world and that generated by our own movements. This task relies on brain circuits that integrate sensory information with an internal model, or expectation, of self-generated movements. The complexity and intractability of many models used to study active sensing means that translating insights from these studies to failures of normal nervous system function remains challenging. Fruit flies (Drosophila melanogaster) actively move their antennae, and my recent work has elucidated a neural circuit that processes mechanosensory information from the antenna. Given the power of Drosophila as a genetic model organism, this project aims to develop the neural circuits controlling and sensing antennal movement as a cellular model for studying principles of active sensing. In the K99 (mentored) portion of this grant, I will identify the cellular location at which self- versus externally-generated mechanosensory signals become differentially represented in the brain. I will make electrophysiological recordings of intracellular activity from 2nd and 3rd order mechanosensory neurons and compare how these two populations encode passive and active movements of the antennae. I will distinguish between these two types of movements using machine learning analysis of simultaneously recorded video data. For the R00 (independent) phase, I will use optogenetics and immunohistochemistry to identify motor neurons that control antennal movement. I will then ask where input from motor neurons impinge on the sensory circuit. Finally, I will test the role of active antennal movements in behavior. By perturbing active antennal movements in freely walking and flying flies, I will directly test how these movements enable different behavioral tasks such as wind orientation and obstacle avoidance. Together, these experiments will identify the cellular basis for active sensing in Drosophila, and their role in goal- oriented behaviors.